1. Field of the Invention
The present invention relates to a positive electrode active material for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery having, in particular, an excellent cycle characteristic.
2. Description of the Related Art
In recent years, along with rapid progress in household electric appliances toward portable and cordless, lithium ion secondary batteries have become commercially available as power sources for compact electronic devices, e.g., lap top personal computers, cellular phones, and video cameras. Regarding the lithium ion secondary batteries, since Mizushima et al. reported the usefulness of lithium cobaltate as a positive electrode active material for the lithium ion secondary batteries in 1980 (“Material Research Bulletin”, vol 15, p. 783-789 (1980)), active research and development have been made on lithium compound oxide, resulting in many proposals until now.
However, a lithium secondary battery by using lithium cobaltate has a problem in deterioration of a cycle characteristic due to, for example, elution of cobalt atoms.
Furthermore, lithium-cobalt compound oxides in which Mg atoms have substituted for a part of Co atoms of lithium cobaltate have been propHsed as positive electrode active materials (refer to, for example, pages 1 and 8 of Japanese Unexamined Patent Application Publication No. 5-54889, pages 2 and 3 of Japanese Unexamined Patent Application Publication No. 2000-11993, and pages 2 and 7 of Japanese Unexamined Patent Application Publication No. 2004-79386).
Regarding the lithium-cobalt compound oxides in the above-described three patent documents, magnesium oxide, magnesium carbonate, or the like is used as a Mg source of a raw material, and regarding lithium secondary batteries including lithium-cobalt compound oxides which contain the resulting magnesium atoms (hereafter may be referred to as Mg atoms) and which serve as positive electrode active materials, it is difficult to obtain satisfactory cycle characteristic.
The present inventor et al. previously proposed to use a lithium-cobalt compound oxide, which was produced by using a MgF2 halide as a Mg source, as a positive electrode active material for a lithium secondary battery (refer to, for example, Japanese Unexamined Patent Application Publication No. 2003-221235, Japanese Unexamined Patent Application Publication No. 2004-339032, Japanese Unexamined Patent Application Publication No. 2006-169048, Japanese Unexamined Patent Application Publication No. 2004-342554, and Japanese Unexamined Patent Application Publication No. 2005-225734).
Furthermore, use of a lithium-cobalt compound oxide, which contains sulfate groups, as a positive electrode active material for lithium secondary battery have been previously proposed (Japanese Unexamined Patent Application Publication No. 2000-21402).
The present inventor made further study on the basis of Japanese Unexamined Patent Application Publication Nos. 2003-221235, 2004-339032, 2006-169048, 2004-342554, and 2005-225734 and found that in the case where a magnesium halide was used as a Mg source, magnesium was present preferentially as a magnesium oxide on surfaces of particles of a lithium-transition metal compound oxide produced by firing through the action of an anionic component constituting the raw material magnesium compound so as to suppress elution of cobalt ions which had an influence on a cycle characteristic and, thereby, a lithium secondary battery including the lithium-transition metal compound oxide as a positive electrode active material exhibited excellent cycle characteristic. It is expected that the performance of the lithium secondary battery is further improved.